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SAN FRANCISCO — President Obama and New York Mayor Michael Bloomberg have signed on to SoundCloud to share their speeches. Comedian Adam Carolla, broadcaster Ryan Seacrest and musicians Kelly Rowland and Beyoncé also post their work on SoundCloud.

Its co-founder and CEO, Berlin-based Alexander Ljung, describes SoundCloud as "YouTube for audio," a place where folks can listen to homegrown bands, spoken-word podcasts and comedy. Thanks to the explosion in smartphones and tablets, and interest in all things audio, his site is on a roll.

SoundCloud now has 40 million registered users, up from 10 million in 2011, but most users don't bother registering. The website reaches 200 million unique listeners around the globe on the Web, mobile devices and on social-media sites. "That's 8% of the current online population, or 20 times the size of Sweden," said Ljung on a recent visit here. "As large as that is, with 8%, that means there's 92% missing. We have a lot of work to do."

POSTING IS EASY AND FREE — UP TO A POINT

The idea behind SoundCloud is exactly like YouTube: Anyone can post their audio and share it really easily on the Web, on their blogs and websites, Twitter, Facebook, Tumblr and other social media. If the audio isn't produced, SoundCloud has a record button on its upload page for anyone to produce audio directly from their built-in webcam microphone found on most laptops and desktops.

There is a big difference between the two sites. YouTube doesn't charge to post videos, while SoundCloud only allows for two hours of audio for free. After that, the cost is $38 yearly for up to four hours, or $130 yearly for unlimited.

Some 12 hours of audio are uploaded to SoundCloud every minute.

The company is not yet profitable. SoundCloud won't reveal how many pay to subscribe, but Forbes estimates it at 5% of the registered users.

Some $45 million in funding has been raised for the company by a variety of venture capital firms, including heavy hitter Kleiner Perkins Caufield & Byers, the company behind Twitter and Flipboard, among others.

SoundCloud was launched in 2008 by Sweden natives Ljung and Eric Wahlforss, but it wasn't until a re-design in 2012 that the site started seeing its massive growth. That re-design made it much easier to share audio clips across the Web and on social-media platforms.

A SHARING MACHINE

Remember the early days of audio online — when you had to download the Real Player or Windows Media Player to listen? The days of downloading before listening are over.

Larry Magid, who files daily tech reports for CBS News that then get played on CBS radio stations as well, has his work shared by CBS on SoundCloud. He likes that from there, it can go on Facebook, Twitter, his website — all over the Web. "That's the way in which media people can get the kind of exposure we need in today's environment."

Online audio has seen a huge boost in the last two years. "A lot of people thought podcasting had died," says Tim Akimoff, the digital content editor for Chicago public radio station WBEZ, which produces the popular This American Life show for National Public Radio.

"But then ... people started to realize — I can't always watch a video. But you can always listen." WBEZ began putting its shows on SoundCloud in 2013, and has seen large audience spikes.

The station's Worldview, a daily global affairs talk show, has been on the air for 19 years in Chicago, attracting average numbers. "Suddenly, the back catalog goes on SoundCloud, and the show is averaging 50,000 plays a day on SoundCloud," says Akimoff. "Last month the show had 1.2 million plays. We've been astounded by that."

I began posting audio of USA TODAY-produced interviews on SoundCloud in the preparation of this article and have seen enormous feedback from the worldwide community, picking up thousands of followers in less than a month.

Podcast production company Earwolf, which distributes shows from comedian Jeff Garlin and Kevin Pollak, switched to SoundCloud in April as a way to get a larger Web audience for its 19 shows. Via SoundCloud, the shows get distributed as well to Apple's iTunes, where "80% of all podcast listening still happens," says Lauren Tyree, Earwolf's studio manager. "It's a great vehicle for us."

Book publisher Simon & Schuster posts audio previews of its spoken books on SoundCloud, primarily because the embeddable player is so easy to work with that most of the authors can easily post chapters and share them through social media, says director of marketing Sarah Lieberman.

"We want to give authors content to share in a real easy way," she says.

MUSIC IS A SOUNDCLOUD STAPLE

But it's not just spoken word. SoundCloud was created by musicians (Ljung is a former studio engineer and Wahlforss is a former electronic musician) and a good 50% of the content on the site is both homegrown music by musicians and professional content posted by the likes of Beyoncé and Rowland.

Musician Melissa B says SoundCloud has done wonders for her fledgling career. She uses Twitter to interact with fans, and directs them to SoundCloud to hear her latest work. "People start spreading the news from there," she says. "It gives you the freedom to communicate with fans. It's been the best platform for me."

Fred Wilson, a partner with Union Square Investors, one of SoundCloud's backers, says that even with the huge online audience that has flocked to the site, these are early days. "It's only getting started," he says. SoundCloud will become "as big as the Internet itself."